Superplastic forming and diffusion bonding (SPF/DB) techniques are becoming widely used for the production of many types of hollow metallic "sandwich" type fabrications with an internal structure, such as airframe and aeroengine components or heat exchanger matrices. In some such components it is desirable to ensure that their internal structures are hermetically sealed against ingress of corrosive agents from the environment, such as water vapour and oxygen. In such cases the hollow structure can be evacuated after the superplastic forming process has been completed, or alternatively purged and filled (perhaps under pressure) by a suitable inert gas, such as nitrogen or argon.
Briefly, the usual way of achieving superplastic forming of the interior structure of a hollow component manufactured from a number of metallic layers is to diffusion bond the layers to each other in selected places to define the areas in the interior structure where superplastic forming will not occur. Thereafter the component is heated to a suitable temperature in a furnace and inflated by pumping an inert gas into its interior at high pressure until a desired amount of superplastic deformation occurs. During inflation the component is restrained between dies to prevent over-inflation and achieve the desired external shape. Inflation is conveniently achieved by means of at least one small diameter pipe which is welded into the structure at a non-critical point, e.g., where material is to be trimmed off to provide the finished external shape of the component. To ensure minimum interference with the structure of the component by the pipe, we prefer that the pipe projects only a little way into the component, connection to the prospective hollow interior being completed by means of a gas entry passage in the form of at least one slot or groove in one or more of the layers. The same pipe can then be used as a means for final evacuation or pressurisation of the hollow interior. Thereafter, the gas entry passage can be sealed and the pipe removed.
We have found that the cheapest and most convenient way of sealing a gas entry passage while maintaining a vacuum or inert atmosphere in the hollow internal structure is to make a fusion weld pass across the passage through the outer layer or layers of the component.
However, a problem arises when the weld pass is made. Although the gas entry passage is initially of very small height in the welding direction, so that the molten weld material might be expected to bridge it satisfactorily during the short interval before it solidifies, during superplastic forming the passage expands somewhat under the high superplastic forming temperatures and pressures. Consequently, difficulty is experienced in ensuring that the weld material satisfactorily bridges the passages thickness, particularly when the interior of the component is evacuated or pressurised. In the former case, the molten weld material tends to be sucked into the interior of the structure and in the latter case it tends to be blown out towards the pipe. Thus, there is a danger that the weld produced will be porous and the component will not be properly hermetically sealed.